Literary tastes are as heritable as other human phenotypes: Evidence from twins’ library borrowing

Social science research argues that differences in individuals’ literary and cultural tastes originate in social environments. Yet, it might be that these differences are partly associated with genetic differences between individuals. To address this possibility, we use nation-scale registry data on library borrowing among Danish twins (N = 67,900) to assess the heritability of literary tastes. We measure literary tastes via borrowing of books of different genres (e.g., crime and biographical novels) and formats (physical, digital, and audio) and decompose the total variance in literary tastes into components attributable to shared genes (heritability), shared environments (social environment shared by siblings), and unique environments (social environments not shared by siblings). We find that genetic differences account for 45–70 percent of the total variance in literary tastes, shared environments account for almost none of the variance, and unique environments account for a moderate share. These results suggest that literary tastes are approximately as heritable as other human phenotypes (e.g., physical traits, cognition, and health). Moreover, heritability is higher for socioeconomically disadvantaged groups than for advantaged groups. Overall, our results suggest that research should consider the role of genetic differences in accounting for individual differences in literary and broader cultural tastes.


Assumptions and Limitations in the ACE Model
The first assumption in the ACE model, the Equal Environments Assumption (EEA), is that MZ and DZ twins share rearing environments to the same extent.Some research argues that, compared to DZ twins, MZ twins are more often in contact and treated more similarly by parents.If true, this would violate the EEA, leading to greater (non-genetic) similarity between MZ twins and thus to inflated estimates of A. Research that addresses the EEA finds it to hold in most cases and, when violated, leads to moderate bias [1,2].Violation of the EEA creates further bias if parents treat same-sex twins more similarly than opposite-sex DZ twins because of sex stereotypes.While this assumption might not hold in the main analyses, comparing same-sex twins with samesex closely spaced siblings separately by sex yields similar results (S1_File S4).
The second assumption in the ACE model is that there is no assortative mating based on genotype, i.e., (future) parents not selecting into couples based on genetic similarities.If parents are more genetically similar than random individuals, DZ twins share more than 50% of segregating genes, as assumed in the ACE model.In that case, the ACE model overestimates the role of shared environments (C) [3,4].It is possible to test the implications of this assumption by varying twins' assumed genetic correlation when estimating the ACE model.However, as our estimates of the role of shared environments are consistently (close to) zero, it is unlikely that a violation of this assumption affects our results.
The third assumption in the ACE model is that genetic effects are additive, i.e., there is no genetic dominance and no gene-gene interactions.Genetic dominance and gene-gene interactions might manifest in rMZ, i.e., correlations between MZ twins' literary tastes, being more than twice that of rDZ, i.e., correlations between DZ twins' literary tastes.We can address this assumption based on model fit statistics.S1_File S3 shows that, in many cases, the intra-class correlation among same-sex twins is more than twice that of the intra-class correlation of opposite-sex twins.However, the intra-class correlation for same-sex twins is not twice that of same-sex closely spaced siblings, and results are robust to using closely spaced siblings as substitutes for DZ twins (see S1_File S4 and S5a-S5b).Finally, S1_File S7 summarizes results from ADE models that, unlike the ACE model, incorporate non-additive, genetic dominance as well as additive genetic influences [5].Results from ADE models lead to the same substantive conclusions as the ones we present in the main analysis.
The fourth assumption in the ACE model is that there are no gene-environment interactions (GxE).GxE implies that some environments are more conducive to the expression of genetic predispositions than

Figure S1 .
Figure S1.Share of individuals that has taken out at least one book from the adult book collection in 2021, by family disposable income and wealth percentile rank.The figure shows that the likelihood of taking out at least one book from the library increases with family disposable income and wealth, but also that differences in library use are not very large.

Table S1 .
Polychoric correlations between library borrowing and self-reported cultural participation.
Notes: We matched registry data on library borrowing in 2021 with survey data on cultural participation from Kulturvaneundersøgelsen (a nationally representative survey carried out by Statistics Denmark).We used the most recently available data in Kulturvaneundersøgelsen (2019-2021).All variables are dichotomous.The variables in the registry data measure if an individual borrowed a book of a particular format in 2021.The variables in the survey data measure if an individual reported participating in a cultural activity within the last three months.N=20,812.The table shows positive correlations between individuals' library borrowing, self-reported reading behavior, and whether they have visited a library.It also shows positive correlations between library borrowing and participation in different cultural activities.

Table S2 .
Polychoric correlations between borrowing library books of different genres and selfreported literary tastes.We matched registry data on library borrowing in 2021 with survey data on cultural participation from Kulturvaneundersøgelsen (a nationally representative survey carried out by Statistics Denmark).We use data from two rounds of Kulturvaneundersøgelsen that asked participants about their literary tastes (2019 Q1 and 2021 Q3).Polychoric correlations.All items are dichotomous.The variables in the registry data measure if an individual borrowed a book of a particular genre in 2021.The variables in the survey data measure if an individual reported reading literature of a particular genre (multiple genres allowed).N=5,680.The table shows positive correlations between the genres of books individuals borrow from the library and the genres of books they report reading.

Table S3 .
Intraclass polychoric correlations in (type of) library borrowing in three populations: Same-sex twins, opposite-sex twins, and same-sex closely spaced siblings.
a P value from two-sided Z test.Tests whether A estimate is statistically different from A estimate for any loan of adult books.*Difference statistically significant at P<0.05.

Table S5a .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by sex and format and using closely spaced same-sex siblings as substitutes for dizygotic twins.

Table S5b .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by sex and format and using closely spaced same-sex siblings as substitutes for dizygotic twins.
a P value from two-sided Z test.Tests whether A estimates are statistically different for men and women.No differences are statistically significant at P<0.05.

Table S6 .
Results from ACE models.Share of variance in borrowing any book from the library attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by sex and age group.Models use closely spaced same-sex siblings as substitutes for dizygotic twins.
a P value from two-sided Z test.Tests whether estimates of A are statistically different when comparing the youngest age group to older age groups among men and women separately.*Difference statistically significant at P<0.05.

Table S7 .
Results from ADE models.Share of variance in (type of) library borrowing attributable to additive genes (A), non-additive genes (D), and unique environments (E), as well as total heritability (A+D).Data from 2021.

Table S8 .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Pooled data from 2020 and 2021.
a P value from two-sided Z test.Tests whether A estimate is statistically different from A estimate for any loan of adult books.*Difference statistically significant at P<0.05.

Table S9 .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Data from 2021.
a P value from two-sided Z test.Tests whether A estimate is statistically different from A estimate for any loan of adult books.*Difference statistically significant at P<0.05.

Table S10 .
Results from ACE models.Share of variance in highest completed education and individual disposable income attributable to shared genes (A), shared environments (C), and unique environments (E).

Table S11 .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by age group and fiction genre.
a P value from two-sided Z test.Tests whether estimates of A are statistically different when comparing the youngest age group to older age groups.*Difference statistically significant at P<0.05.

Table S12 .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by age group and nonfiction genre.
a P value from two-sided Z test.Tests whether estimates of A are statistically different when comparing the youngest age group to older age groups.*Difference statistically significant at P<0.05.

Table S13 .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by age group and format.P value from two-sided Z test.Tests whether estimates of A are statistically different when comparing the youngest age group to older age groups.*Difference statistically significant at P<0.05.

Table S14b .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by level of education.

Table S15a .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by level of income.
a P value from two-sided Z test.Tests whether estimates of A are statistically different when comparing individuals whose income is below and above the median.*Difference statistically significant at P<0.05.

Table S15b .
Results from ACE models.Share of variance in (type of) library borrowing attributable to shared genes (A), shared environments (C), and unique environments (E).Separate models by level of income.
a P value from two-sided Z test.Tests whether estimates of A are statistically different when comparing individuals whose income is below and above the median.*Difference statistically significant at P<0.05.

Table S16 .
Share and count of borrowers across full population, females and males, and loan genre/types.

Table S17 .
Share and count of borrowers across education and income, and loan genre/types.